1. Field of the Invention
The present invention relates generally to current collectors used in electrodes of electrochemical cells. More particularly, the present invention relates in one embodiment to a current collector that can deform compliantly during electrode manufacturing and cell operation, thereby avoiding electrode deformation and delamination.
2. Description of Related Art
Compact electrochemical cells or batteries, such as those used to power implantable medical devices, are comprised of an anode and cathode contained within a casing and activated by an electrolyte. Either or both electrodes may include respective electrode active materials that are contacted with opposed first and second sides of a conductive current collector. The current collector is typically formed of a metal screen. Depending upon the cell configuration, the current collector may be connected directly to the casing or, to a terminal wire that exits through and is insulated from the cell casing.
From the beginning of the manufacturing process when electrode active material is contacted to a current collector to form an electrode to the end of cell life, stresses may arise in the electrode. These stresses tend to deform the electrode and cause delamination of the electrode active material from the current collector. Both effects are undesirable and can degrade performance and life of the cell.
Thus, a need exists for a battery current collector that has a more physically compliant structure, which can serve to relieve stresses that would otherwise occur in a non-compliant current collector screen. This need has become more evident in certain cathode structures comprised of materials that change shape, i.e., expand, ‘relax’, etc., during and after the electrode farthing process. Such cathodic structures, when formed, essentially become a composite matrix having a current collector or collectors in combination with one or more types of cathode base materials typically structured in layers. The initial physical form of such cathodic materials before assembly can be sheet, powder, granular, etc.; when formed they essentially become a solid.
Typical prior art current collectors are comprised of a metal screen having a regular pattern of repeating geometric openings or perforations. The shape of such openings may be that of a diamond, square, circle, hexagon, etc. As such, the current collector is essentially the network-like structure remaining after the pattern of openings has been established. Various methods are used to manufacture typical current collectors, such as “punching-stretching” for expanded metal, punching for perforated plate, fine blanking, chemical etching, weaving (such as in wire cloth manufacture), laser cutting, and electroforming.
In addition to their primary function of providing conductivity from the active material to the associated terminal for the cell circuit, current collectors may serve as structural reinforcement in electrodes, particularly cathodes. However, in some cell chemistries, combinations of certain cathode materials formed in a composite matrix contacted to a current collector of the typical, prior art kind have been problematic. In such cathodic structures, conventional current collectors are not able to move sufficiently with the expansion of the cathodic base materials during and after the manufacturing process. The resultant problem manifests itself via a physically constraining effect produced by the one or more embedded current collectors. Thus, stresses develop between the cathode active material and the current collector.
Such stresses may be analogous to the condition present in bimetallic springs used to produce movement of a mechanism. Bimetallic springs are a composite structure formed of at least two components having different coefficients of thermal expansion. Such stresses work as intended in bimetallic springs, since the components remain sufficiently flexible throughout a given temperature range and have a relatively high tensile strength. However, when a similar expansion occurs within certain cathode assemblies, the self-induced stresses can significantly distort or even fracture the cathode assembly, causing delamination of the cathode active materials from the current collector. For such cathode structures that are problematic, the expansion rates of the active material and the current collector are significantly different during and after the assembly process. In the case of fracture, the relatively low tensile strength of the cathode active material is a factor. Hence, there remains a need for a cathode current collector that is more compliant during and following the cathode assembly process than is presently available.